Build and test also with Java 18

Author: bwaldvogel

.github/workflows/gradle.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        java: [ '8', '11', '17' ]
+        java: [ '8', '11', '17', '18' ]
 
     steps:
     - uses: actions/checkout@v2